Ccna 00-301, Volume Official

0.

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Figure M-2

  Creating Subnet Blocks by Adding 1 in the “Just Left” Octet

Technet24

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APPENDIX N

Variable-Length Subnet Masks

NOTE

  This appendix contains an entire chapter that was published as a chapter in one 

of the past editions of this book or a related book. The author includes this appendix with 

the current edition as extra reading for anyone interested in learning more. However, note 

that the content in this appendix has not been edited since it was published in the earlier 

edition, so references to exams and exam topics, and to other chapters, will be outdated.  

This appendix was previously published as Chapter 22 of the book CCENT/CCNA ICND1 

100-105 Official Cert Guide

, published in 2016.

IPv4 addressing and subnetting use a lot of terms, a lot of small math steps, and a lot of 

concepts that fit together. While learning those concepts, it helps to keep things as simple 

as possible. One way this book has kept the discussion simpler so far was to show examples 

that use one mask only inside a single Class A, B, or C network.

This chapter removes that restriction by introducing variable-length subnet masks (VLSM). 

VLSM simply means that the subnet design uses more than one mask in the same classful 

network. VLSM has some advantages and disadvantages, but when learning, the main chal-

lenge is that a subnetting design that uses VLSM requires more math, and it requires that 

you think about some other issues as well. This chapter walks you through the concepts, the 

issues, and the math.

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2    CCNA 200-301 Official Cert Guide, Volume 1